1. Field of the Invention
This invention relates to a finder optical system for a single-lens reflex camera.
2. Description of the Prior Art
Among TTL-type finder optical systems which utilize the light rays traversing the photo-taking lens as rays for the finder, an optical system employing a relay optical system allowing the inverted image formed by the photo-taking lens to be observed as an erect image has been proposed. In this finder optical system, an erect image can be obtained by re-forming the primary image, which is an inverted image formed by the photo-taking lens, into a secondary image by means of a relay optical system. However, because a relay optical system requires a considerably lengthened light path, mounting a finder optical system with a relay optical system into a camera has entailed the necessity of increasing the height or depth of the camera.
A trimming system has also been recently proposed. As shown in FIG. 6(a), a trimming system is a system wherein trimming information that specifies a certain area A (trimming area) of film 5 is stored in a prescribed location 51 of the film at the time of photographing. Based on the trimming information, only the trimming area A is enlarged and printed on the photographic paper shown in FIG. 6(b) at the time of printing. (This mode in which trimming information is stored is hereinafter referred to as the "pseudo focal length mode".) Where the above-referenced finder optical system is mounted in a camera equipped with a pseudo focal length mode, it is possible to use the variable power of the relay optical system in order to enlarge the trimming area to an area as large as the finder field frame. However, in order to change the magnification in the relay optical system, space to allow movement of the relay optical system on the optical axis is needed, which entails the necessity of further increasing the camera size.
In addition, when a finder optical system other than that referenced above is mounted in a camera equipped with a pseudo focal length mode, the problem described below occurs. Namely, in the case of photo-taking lenses where the location of exit pupil changes (i.e., exchangeable lenses and zoom lenses), if the power is changed in the finder optical system, vignetting may occur depending on the location of the exit pupil of the photo-taking lens (details are explained in the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS). This is because the position of the entrance pupil of the finder optical system changes with the change of power; so far, no prior art is known to consider this point.
Further, let us examine the conventional light-measuring optical system of a single-lens reflex camera. Normally, the light-measuring optical system is structured so that a portion of the light rays transmitted through the finder optical system is utilized; however, it has been impossible to perform precise light measurement while maintaining the brightness of the finder image (details are explained in the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS).
Finally, let us examine the in-finder display of a camera equipped with a pseudo focal length mode. It has already been mentioned that the variable power of the finder optical system is exploited in a camera equipped with a pseudo focal length mode. Because an in-finder display is generally performed by means of a display device located close to the primary image, if the magnification of the finder optical system is changed, vignetting may occur in the periphery of the display, or the display may not be seen in an appropriate location and/or size in relation to the field frame. On the other hand, the magnification of the display of the focus detection area and light-measuring area, which exist in fixed relation to the primary image, must change in response to the change in magnification of the finder optical system.